PTC means positive temperature coefficient, which refers to materials that experience an increase in electrical resistance when their temperature is raised.
Polymers can be made electrically conductive by dispersing suitable amounts of conductive particles such as carbon black or fine metal particles. Some of the above polymeric compositions exhibiting positive temperature coefficient (PTC) behavior and a device using the same have been used in many applications, especially in electronic industries, including their uses as constant temperature heaters, over current regulators, and low-power circuit protectors. A typical use is that the amount of the current passing through a circuit, which is controlled by the temperature of a PTC element forming part of the circuit.
There are two major ways to produce such PTC compositions, the one is melt-extruding technology and the other is ink/coating technology. The melt-extruding technology is most popularly applied but the resulting articles may be inflexible and are generally unsuitable for configuration into the intricate or very thin shapes often desirable for use on flexible substrates or printed circuit boards. Over recent years, there has been particular interest in the ink/coating technology to produce polymeric PTC compositions. Reference may be made, for example, to U.S. Pat. Nos. 4,628,187, 5,181,006, 5,344,591, and 5,714,096, Japanese Patent Publication Nos. 2008293672, 2009151976, and 2009199794. For these inks/coatings, the polymer resins are dissolved in suitable solvents as the binders and the conductive particles are dispersed in the binders to obtain the inks/coatings.
Various polymeric PTC compositions have been developed, however, most PTC compositions exhibit Negative Temperature Coefficient (NTC) characteristics immediately after the PTC characteristics. This change from PTC behaviour to a strong NTC behaviour is often undesirable, which may cause self-burning in some cases. FIG. 1 shows a typical temperature-resistance curve of the PTC composition described above. It indicates that when the temperature T, given on a horizontal axis, is in excess of 70° C., a PTC ratio RT/R-25 is reduced, the PTC ratio being a ratio between a resistance R-25 at a temperature of 25° C. and a resistance RT at a certain temperature T. The reducing resistance leads to an excessive current flows and the heating element is burned out. Therefore, the NTC area is called safety risks area.
Efforts have been undertaken to reduce or eliminate the NTC effect. As disclosed in U.S. Pat. No. 5,227,946 and European Patent EP 0311142, in polymeric PTC compositions reduction of the NTC effect has been achieved by cross-linking the material. Most effective is post-cross linking after the forming step either by gamma radiation or accelerated electrons. Cross-linking in the melt also erases the NTC effect but negatively affects the PTC amplitude. In addition, the step of cross-linking the material increases the time and production costs for manufacturing the PTC composition.
U.S. Pat. No. 8,496,854 discloses a method to reduce the NTC effect without cross-linking the material. Their PTC compositions include a thermoplastic base resin, an electrically conductive filler and particles of a polymeric additive dispersed in the PTC composition; wherein the polymeric additive has a melting or softening temperature greater than the melting temperature of the thermoplastic resin, which help reduce the NTC effect. The above PTC composition was produce by melt-extruding technology not like the ink/coating technology used in the present invention. In addition, the NTC effect is only reduced and not completely eliminated by the above method.
The present invention provides a method to completely eliminate the NTC effect of the PTC composition produced by ink/coating technology.